The use of optical fibers in communications is growing at an unprecedented rate. Low loss optical fibers which are produced by any one of several techniques may be assembled into ribbons which are then assembled into cables, or stranded into cables, or they may be enclosed singularly in a jacket and used in various ways in a central office, for example.
In order to assure that the low loss optical fibers which are produced today are not diminished in their effectiveness in systems, the fibers must be connected through intermateable connectors which preserve those low losses. For optical fiber ribbons, connectors comprise grooved chips which hold a plurality of fibers of one ribbon in alignment with fibers of another ribbon. Such a connector is shown for example in U.S. Pat. No. 3,864,018 which issued on Feb. 4, 1975 in the name of C. M. Miller.
For single fiber cables, connections may be made through a connector which is referred to as a biconic connector. See, for example, an article entitled "Interconnection for Lightguide Fibers" which was authored by T. L. Williford et al. and which appeared in the Winter 1980 issue of the Western Electric Engineer beginning at page 87. That connector includes a coupling having a housing in which is mounted a biconic alignment sleeve. The sleeve includes two truncated, conically shaped cavities which communicate with each other through a common place which has the least diameter of each cavity. Each of two optical fibers to be connected is terminated with a plug comprising a primary pedestal or truncated, conically shaped end portion which is adapted to be received in one of the cavities of the sleeve. Each fiber extends through the plug in which it is mounted and has an end which terminates in a secondary pedestal of the plug. The plug is held in a cap having an externally threaded portion adapted to be turned into an internally threaded entrance portion of the housing. At least portions of the conically shaped surfaces of the plug and of the sleeve serve as alignment surfaces and are intended to be conformable. The plug is urged into seated engagement with the wall defining the cavity in which it is received while the cap is turned into the housing.
Minimal loss between the connected fibers is achieved when the optical fibers which are terminated by the plugs are aligned coaxially and when the fiber end faces, each of which is planar, contact in a common plane. Considering the size of the fibers, for example one with a core diameter of 8 microns and a cladding diameter of 125 microns, the task of providing comformable, conical plug and sleeve surfaces in order to meet alignment and end separation requirements is a formidable one.
The alignment sleeves as molded are checked for accuracy by measuring the distance between reference circumferences of the walls of opposing cavities. If the distance is too long, the plugs may seat within the cavities, but the end separation of the fiber end faces is too great. One the other hand, if the distance is too short, the secondary pedestals touch, but there is insufficient contact between the alignment surfaces. Further, if the fiber end faces contact each other prior to seating the conformable portions of the alignment surfaces of the plugs, the fibers within the plugs may become misaligned or the fiber end faces may become damaged. Also important is a plug taper length which is defined as that distance from a reference circumference on the plug boundary to the terminated fiber end face which is the end face of the secondary pedestal. If the taper length is too long, the secondary pedestals may touch but there is not contact between the conforming surfaces; if it is too short, the plugs seat within the cavities of the sleeve, but the end faces of the fibers are spaced apart by too great a distance.
One problem with these kinds of connectors relates to the mounting of the plugs within the sleeve. In some installations, it becomes very difficult, if not impossible, to hold the cable while turning the cap into the housing of the coupling. If the plug is not held while the cap is turned into the housing in which the sleeve is disposed, the plug will turn the cap. If the plug turns, the end face of the plug and hence of the optical fiber terminated therewith may abrade against the plug and optical fiber already in the coupling, possibly causing damage to the optical fiber.
This problem may be overcome by causing the taper length to be such that a gap between the end faces is caused to exist. However, this results in increased insertion losses. These losses may be reduced by providing a quantity of optical index matching material within the cavities of the sleeve, after which the fibers are pushed into the cavities until their end faces engage the conically shaped walls to align the fibers and to place their end faces in close adjacency. The index matching material helps to reduce the transmission loss notwithstanding the fact that the end faces are not contacting. Although this arrangement may provide an adequate connection, it depends on an additional medium which may introduce contaminants at the junction of the optical fibers.
Another problem in attempting to provide a solution to the problem of unintended plug rotation relates to compatibility. With many biconic connectors already in use, it would be imprudent to provide plugs which overcome the problem of unwanted rotation but which are not compatible with sleeves already in use.
Seemingly, the prior art has not yet offered a simple solution to the problem of mounting conically shaped plugs which terminate optical fibers in a biconic coupling in a manner which avoids end face abrasion of the fibers. The sought-after connector should be one which provides for improved insertion loss and performance repeatability. Desirably, the plug and the sleeve of the sought-after connector are ones which are compatible with connectors that already are in use in the field.